What is the default Random Ruler Placement game rule in CK3?

Off: Rulers have their historical Titles and Realm. (patch 1.20.1.2)

Does changing the Random Ruler Placement rule disable achievements?

Yes for “On” and “On (Conversion)”: the file gives them the flag blocks_achievements. The other settings keep achievements on.

Game rule · Game Modes

Random Ruler Placement

Off · default
On ✕
On (Conversion) ✕

Random Ruler Placement is a game rule in Crusader Kings III (Game Modes) with 3 settings: Off (the default), On and On (Conversion). “On” and “On (Conversion)” disable achievements. Patch 1.20.1.2, from game/common/game_rules/00_game_rules.txt.

This section as JSON: /raw/game-rules.json

Quick answers (2)

What is the default Random Ruler Placement game rule in CK3?: Off: Rulers have their historical Titles and Realm. (patch 1.20.1.2)
Does changing the Random Ruler Placement rule disable achievements?: Yes for “On” and “On (Conversion)”: the file gives them the flag blocks_achievements. The other settings keep achievements on.

3 settings

Off
Default
Rulers have their historical Titles and Realm.
The script checks it once, in 1 place
On actions (the game's own hooks) 1
- on_game_start_after_lobby
Every has_game_rule = random_ruler_placement_off in game/common and game/events, by the top-level object it sits in.
random_ruler_placement_off
On
Disables achievements
Rulers are randomly placed on the map. The Realms are largely the same, but Titles are held by different rulers.
random_ruler_placement_on
On (Conversion)
Disables achievements
Rulers are randomly placed on the map. They convert to the historical holder's Faith.
The script checks it once, in 1 place
Scripted effects 1
- game_rule_random_ruler_placement_shuffle_list_effect
Every has_game_rule = random_ruler_placement_on_conversion in game/common and game/events, by the top-level object it sits in.
random_ruler_placement_on_conversion

Source: game/common/game_rules/00_game_rules.txt line 1067 (key random_ruler_placement), modifiers from game/common/modifiers, names from the English localisation; Crusader Kings III 1.20.1.2.